BAW resonators are applicable to frequency bands of a few hundred MHz to more than 10 GHz, and examples of their application include the use of BAW resonators in the configuration of a radio-frequency filter in a mobile communication system such as mobile telephones (see, for instance, Electronics Letters, Vol. 35, pp. 794-795, 1999).
Known techniques for downsizing BAW resonators and filters include one by which BAW resonators are divided in the perpendicular direction (see U.S. Pat. No. 7,038,559 B2).
Also, U.S. Pat. No. 7,019,605 B2 contains the following statement regarding U.S. Pat. No. 5,587,620 quoted in it, with reference to FIG. 3: “The SBAR shown in FIG. 3 can be regarded as being composed of two FBARs, one stacked on top of the other. One of the FBARs is composed of piezoelectric layer 22 sandwiched between electrodes 24 and 26. The other of the FBARs is composed of piezoelectric layer 42 sandwiched between electrodes 26 and 44. Electrode 26 common to both FBARs provides close coupling of acoustic energy between the FBARs.”